The efficient conversion of cellulose to a carbohydrate such as glucose which can be used as a starting material for other products, or for the production of food for humans, has been an object of intense research activity for many years.
The digestive systems of human beings and many animals cannot break down and absorb nutrient value from cellulose. Only ruminating animals such as cows, which have four stomachs, are capable of breaking down and digesting the complex and highly stable cellulose molecule. Cellulose, if it could be conveniently and economically reduced to a readily human digestible product, would open up a completely new food source for human beings.
At the present stage of the biological development of mankind, petroleum hydrocarbons are utilized as starting products for many materials used by humans, for example, polymers such as polyethylene and polystyrene, gasoline for automobiles, and man-made fibres for clothing. Unfortunately, petroleum is a depleting non-renewable resource. Coal can be used as a substitute, but it also is a non-renewable resource. Cellulose, which is the backbone of most flora and occurs naturally on a renewable basis, if it could be efficiently and economically broken down into manageable carbohydrate, could provide a renewable resource which conceivably could replace, at least in part, petroleum or coal as a basic starting block for mankind's needs.
The biological conversion of cellulose to glucose requires the activity of three types of enzymes. The insoluble substrate is attacked first by extracellular cellulases, of which there are two major types: the endo-1,4-.beta.-glucanases (EC 3.2.1.4), or Eng; and the exo-1,4-.beta.-glucanases (cellobiohydrolases, EC3.2.1.91), or exoglucanases (Exg) (Mandels, 1982; Gilbert and Tsao, 1983). The cellobiose resulting from the action of these enzymes is converted to glucose by .beta.-1,4-glucosidases (EC3.2.1.21) or cellobiases (Mandels, 1982; Gilbert and Tsao, 1983). The cellobiases are usually intracellular, but some are extracellular (Mandels, 1982).
The inventors have been characterizing the cellulase system of the bacterium, Cellulomonas fimi (Gilkes et al., 1984 a,b; Langsford et al., 1984) by cloning the genes determining all of the components of the system. It is intended to use the cloned genes to characterize the components of the system through nt sequencing and prediction of aa sequences from the nt sequences. Cellulase systems tend to be very complex, and their components can prove difficult to purify by biochemical methods (Mandels, 1982; Gilbert and Tsao, 1983). Gene cloning can serve to identify components when other methods have failed (Gilkes et al., 1984 a,b). The product of a cloned cellulase gene can be produced in a cellulase-free background in an appropriate host microorganism (Whittle et al., 1982; Gilkes et al., 1984 a; Skipper et al., 1985). The individual enzymes then can be used to reconstitute the original cellulase system. Appropriate mixing experiments would help to elucidate the interactions of the components and to define their roles in the degradation of cellulose.